500 Naturwissenschaften und Mathematik 570 Biowissenschaften; Biologie 577 Ökologie
Like many urban centers did, also Berlin developed along water courses. With the growth of the city and increasing industrialization, natural rivers were modified for navigation purposes and artificial waterways were built. The present network of waterways is almost continuously embanked, water levels are regulated and many additional uses exist. Nonetheless, it is an aquatic habitat and thereby subject to regulations of the EU Water Framework Directive (WFD).
To improve the ecological conditions according to WFD requirements, artificial shallow littoral zones were built during two larger construction projects in the River Spree and the Teltow Canal. Sheet pile walls with frequent openings ought to reduce navigation-induced hydraulic disturbances at the banks and thereby create new aquatic habitats without these impacts.
Within the present study, abiotic conditions at both rehabilitation sites were measured, analyzed and simulated. In addition, the WFD-relevant quality elements macrophytes, benthic invertebrates and fish were sampled. In parallel, wave-exposed banks were sampled to better account for rehabilitation effects.
While exposed banks had only sparse vegetation, protected banks benefited and developed abundant vegetation within five years, indicating a good ecological status. At the same time flow resistance and residence times increased. Due to lowered connectivity and the oxygen engineering capabilities of vegetation, hypoxic conditions were caused, which in turn influenced aerobic organisms.
Benthic invertebrates were indeed less abundant at wave-protected sites. None\-theless, most of them were native taxa and the community was diverse and rich in endangered taxa. In contrast, wave-exposed substrates were populated by high densities of non-native taxa, common for European waterways. However, the increase in diversity and the absence of non-native taxa did not improve the WFD assessment results which were generally poor.
The fish community was dominated by roach and European perch and deviated strongly from good conditions. Regardless, fish benefited from habitat complexity. Therefore the artificial shallow banks can be suitable habitats. However, together with the appearance of oxygen deficits they lost their suitability for fish.
Generally, both rehabilitation sites led to an improvement of the waterway habitat. The older site showed signs of rapid succession and required morphological changes or maintenance works. Modifications of the openings should be the primary measure to improve the connectivity whereas later on also vegetation removal or dredging could be appropriate.
The available WFD assessment tools cannot detect significant improvements of the ecological conditions. Thus, on one side, adaptations of the assessment tools for local measures and urban waterways are required. On the other side, the present assessment results emphasize the need to create riverine conditions and habitats in the future rehabilitation actions.
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